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More Signs of a Toasty Greenhouse

The Northern Hemisphere's three warmest years in the last 6 centuries were 1990, 1995, and 1997, according to a new climate analysis in tomorrow's issue of Nature. Researchers compiled records from tree rings, ice cores, corals, and other sources to conclude that warming from rising levels of carbon dioxide and other greenhouse gases this century has overwhelmed all other factors influencing climate. The results bolster similar findings from other studies, although uncertainties remain in gauging the swings of Earth's touchy thermostat.

Direct temperature measurements exist for the 1900s and much of the 1800s. For times earlier than that, scientists must devise indirect ways to track temperature. Evidence of warm and cool cycles can be found in several archives in nature, such as the widths of tree rings and the chemical compositions of ice deposits and annual coral layers. But trees, ice, and coral respond not just to temperature but to other factors such as seasonal changes in rainfall.

To calibrate the specific contribution of temperature changes, climatologists examined how much of the fluctuations in growth patterns seen in modern trees, ice cores, and coral reefs can be explained by known temperature records. Then they extended that statistical relationship to natural samples preserved from previous centuries, thus reconstructing a 600-year history of average annual temperatures in the Northern Hemisphere.

The results show that, to a certainty of 99.7%, "none of the previous years could have been as warm as 1990, 1995, and 1997," says team leader Michael Mann of the University of Massachusetts, Amherst. Further analysis suggested that natural events, such as volcanic eruptions and cyclical changes in sunlight intensity, have dictated past warmings and coolings. But the marked upswing in temperatures this century, Mann concludes, could result only from rising greenhouse gases.

The approach taken by Mann's group is "intriguing and promising," says climatologist Gabriele Hegerl of the University of Washington, Seattle. "It does add confidence that climate patterns in the 20th century are broadly warmer" and that greenhouse gases are the culprits, she says. However, Hegerl feels it is too early to pinpoint the 1990s as the most sultry decade, because temperature reconstructions for specific years in past centuries still contain too many potential errors--as do efforts to calculate the average annual temperature over an entire hemisphere.